Fuel cells use the chemical reaction of a fuel with oxygen to form water to generate electrical energy. For this purpose, fuel cells contain, as the core component, the so-called membrane-electrode assembly (MEA), which is a composite of a proton-conductive membrane and an electrode arranged on either side on the membrane (anode and cathode). During operation of the fuel cell, the fuel, in particular hydrogen H2 or a hydrogen-containing gas mixture, is supplied to the anode, where an electrochemical oxidation takes place with the emission of electrons (H2→2H++2e−). A (water-bound or water-free) transport of the protons H+ from the anode chamber into the cathode chamber takes place via the membrane, which separates the reaction chambers from one another in a gas-tight manner and electrically insulates them. The electrons provided at the anode are conducted via an electrical line to the cathode. Oxygen or an oxygen-containing gas mixture is supplied to the cathode, so that a reduction of the oxygen takes place with absorption of the electrons (½O2+2e−→O2−). Simultaneously, these oxygen anions react in the cathode chamber with the protons transported via the membrane to form water (2H++O2−→H2O). Due to the direct conversion of chemical energy into electrical energy, fuel cells achieve an improved efficiency in relation to other electricity generators as a result of the avoidance of the Carnot factor. The cathode reaction represents the speed-limiting element of the fuel cell reaction, inter alia, due to the lower diffusion speed of oxygen in relation to hydrogen.
In general, the fuel cell is formed by a plurality of membrane-electrode assemblies arranged in a stack, the electrical powers of which are added together. A bipolar plate is situated between each two membrane-electrode assemblies of a fuel cell stack, which has channels for supplying the process gases to the anode and the cathode of the adjacent membrane electrode assemblies, on the one hand, and also coolant channels for dissipating heat. Bipolar plates additionally include an electrically conductive material to establish the electrical connection. They therefore have the threefold function of the process gas supply of the membrane-electrode assemblies, the cooling, and the electrical connection.
Bipolar plates are known in different designs. Weight reduction, installation space reduction, and increase of the power density represent fundamental goals in the design of bipolar plates. These criteria are important in particular for the mobile use of fuel cells, for example, for the electromotive traction of vehicles.
US 2005/0058864 A1 (U.S. Pat. No. 6,974,648 B2) and US 2006/0029840 A1 (U.S. Pat. No. 7,601,452 B2) describe bipolar plates for fuel cells, which are constructed from two undulated and interleaved plates. Each of the plates has a meandering profile, so that grooves are formed in each case on both sides, which are delimited by wall-like projections. The two plates have different widths of the grooves or projections formed. Closed channels, which are used as cooling channels, are formed in the interleaved microstructure of the plates. The open channels (grooves) provided on both sides of the microstructure face toward the anode on one side and the cathode on the other side of the adjacent MEAs in the assembled fuel cell stack and are used for the supply thereof with air/oxygen or fuel/hydrogen, respectively.
The bipolar plate described in WO 03/050905 A2 has continuous depressions on one side to form anode channels and continuous depressions on the other side to form cathode channels. Furthermore, the plate has enclosed coolant channels. All channels extend in parallel to one another.